Corrosion of aluminum alloys on vehicles is a complex and costly issue for vehicle manufacturers. Aluminum corrosion is generally associated with joints between parts, hem flanges and trimmed edges. As more aluminum is used in vehicle designs the potential for greater aluminum corrosion is an increasingly important issue. The root cause of aluminum corrosion is complex but is believed to relate to five key elements: surface finishing, paint shop-applied pretreatment, aluminum alloy chemistry, the presence or absence of a sealer, and craftsmanship (placement of adhesive, inner/outer overlap, inner/outer gap, burr size and orientation). All of the above elements must be addressed to achieve superior aluminum corrosion performance.
Manufacturing materials and processes are not solely optimized to provide the best aluminum corrosion performance. New vehicle designs are trending towards deeper draws and stronger alloys that necessitate the need for improved forming manufacturing methods and materials. Aluminum vehicles depend on substantial amounts of structural adhesive that is applied to parts being joined to provide the vehicle with the necessary strength and rigidity to improve noise, vibration and harshness (NVH) performance. The interaction between the aluminum surface, the forming lubes, and the adhesive must be balanced to provide an effective manufacturing process.
All aluminum surfaces normally have a native oxide/hydroxide layer due to the reactive nature of aluminum. The native oxide/hydroxide layer forms very rapidly when the aluminum is exposed to air. This reaction is self-passivating and results in a thin oxide/hydroxide layer. When pretreating an aluminum coil, an alkaline and/or acid etch is performed to remove the native oxide/hydroxide layer. A thin film pretreatment layer (such as Alcoa 951 a trademarked product of Alcoa Aluminum for a vinyl phosphonic/phosphinic acid, polyacrylate solution) may be applied to the newly etched surface. Some of the native oxide/hydroxide layer may reform on the surface before the Alcoa 951 is applied which is then modified by the Alcoa 951 pretreatment.
The general problem addressed is how to deliver superior formability, adhesive bond durability, and corrosion performance on an aluminum vehicle at high production volumes.
The above problems and other problems are addressed by the disclosed method as summarized below.